| Abstract: | The formation and growth of porphyroblasts is treated in detail, with particular reference to garnet. Previous explanations for included fabrics in porphyroblasts, and mechanisms of syntectonic growth leading to sigmoidal inclusion patterns, are shown to be based on invalid assumptions and are unacceptable. The following new mechanism for garnet porphyroblast growth is proposed. During deformation, fluid filled veins open, particularly where micaceous foliae open in folds associated with crenulation cleavages and kink bands. Garnet nucleates in these veins as they develop. The geometry of the veins depends on variability of strain rates, fluid diffusivity and the sequence of progressive deformation. In areas of low strain rate and high diffusivity, long and widely spaced macroscopic garnet veins form. These may be buckled during the continuous deformation process. Conversely, where strain rates are high, and diffusivity is low, short and closely spaced microscopic garnet veins form. Buckling and shear may alter the vein shapes during their formation and sigmoidal veins can result. Veins join and coalesce to form single porphyroblasts, leaving a selvage of quartz and other minerals as an included fabric. The finite geometry of the included fabric ranges from helicitic to sigmoidal, which is interpreted to reflect the local deformation history during vein propagation. The fabric geometry is finalized when the veins fail to respond individually to the deformation due to their coalescence and formation into a single porphyroblast. Continuous vein propagation without significant deformation of the veins could lead to augen textures, whilst a process of continuous vein formation and rotation is shown to explain apparent syntectonic rotations of more than 180°. This new hypothesis has the merit of being very simple, but it does require a new approach. This approach necessarily questions a number of previously well established “facts”, such as pre-, syn-, and post-tectonic textures. It is argued that extreme care must be taken, when relating deformation episodes and metamorphic growth periods. |
